Large steel mills have conventionally made batches of steel alloys in ladles. To make steel alloys, base steel has been combined with various alloying elements, for example, copper, manganese, chromium, zinc, nickel and/or cobalt, in batch ladles having batch sizes typically exceeding 200 tons. It is economically inefficient to produce less than a full batch of a particular grade of alloy steel, in a ladle. Therefore, it has been a common practice of large steel companies not to accept an order of less than one ladle batch, for a particular grade of steel.
Notwithstanding the previous difficulty for large steel companies to produce small order quantities, there has always been some demand for small order quantities of steel alloys by steel consumers. In the past, these smaller orders have been filled by specialty producers using smaller ladles, etc. In more recent years, the demand for smaller order quantities of steel alloys has risen sharply while the demand for larger order quantities has leveled off or fallen. As a result, there is increasing pressure on large steel companies to develop or obtain suitable, economically efficient techniques for producing smaller order quantities (i.e., less than 200-ton batch sizes) of steel alloys.
Another problem faced by large steel companies, particularly associated with intermediate order sizes of one or a few ladles, has been the loss of steel during production transition from one alloy grade to another. After a steel alloy is produced in a ladle, the molten steel is fed to a tundish vessel for purification of the steel and distribution to various casting machines and/or molds. In a typical large steel mill, a single tundish is selectively fed from more than one, and often several, different ladles, each containing a different grade of steel.
When switching a tundish from one ladle to another, the rate of flow into the tundish from the first ladle is gradually lowered until the level of molten steel in the tundish significantly drops. For example, the rate of flow into a 55-ton tundish vessel may be lowered from five tons per minute to two tons per minute until the level of molten steel in the tundish drops to below 25 tons. Once this lower level is achieved, the flow into the tundish from the first ladle is stopped and the flow from the second ladle is commenced, initially at a low flow rate sufficient to just maintain the lower level of molten steel in the tundish.
This low flow rate is maintained until substantially all (i.e. a target percentage) of the molten steel in the tundish has been replaced by steel from the second ladle. Thereafter, the flow to the tundish from the second ladle is gradually increased to, for example, five tons per minute, and the level of steel in the tundish is allowed to rise back to 55 tons. During this transition from the first ladle to the second ladle, a quantity of steel (for example, 20 tons) is lost because this steel is an unsalable mixture of steel from the first and second ladles. In order to more efficiently produce intermediate quantities of steel alloys, there is also a demand for processing techniques which reduce this transition loss in the tundish.